The distinction between an Arrow-junction and a
Y-junction is whether the three edges subtend a total angle of
less or more than .

The dividing case between an Arrow-junction and a
Y-junction is
specifically identified as a T-junction.

Consideration of what such junctions can represent in a view of a
polyhedral model leads to the conclusion that only certain labellings of the
edges at a junction can arise in images of a real scene. These are
illustrated in Fig. 41.